1. Field of the Invention
The present invention relates to a high frequency power amplifier, and more particularly to a push-pull type high frequency power amplifier, and a power distributor and a power combiner for use therein.
2. Description of the Background Art
FIG. 6 is a diagram showing a structure of a conventional push-pull type high frequency power amplifier. In FIG. 6, a conventional power amplifier 9 includes: an input terminal 900; a balun/matching circuit 901; field-effect transistors (FETs) 902a and 902b of the same specification; inductors 903a and 903b respectively connected to drains of the FETs 902a and 902b; an output terminal 904; a distributed constant line 905 connected between the output terminal 904 and the drain of the FET 902a; a series resonant circuit 906a connected between the distributed constant line 905 and the drain of the FET 902a so as to be in parallel connection with the distributed constant line 905 and the FET 902a; and a series resonant circuit 906b connected between the output terminal 904 and the drain of the FET 902b so as to be in parallel connection with the output terminal 904 and the FET 902b. 
The balun/matching circuit 901 inverts the phase of a fundamental wave inputted into the input terminal 900, and outputs first and second signals having fundamental wave components inverted in phase with respect to each other. The first and second signals are respectively amplified by the FETs 902a and 902b. In this case, it is assumed that a wavelength of the fundamental wave is λ, and the distributed constant line 905 has a length corresponding to a ½ wavelength of the fundamental wave, i.e., λ/2. The distributed constant line 905 shifts a phase of a fundamental wave of a signal outputted from the FET 902a by 180 degrees, and then combines the signal with a signal outputted from the FET 902b. A resultant signal is outputted from the output terminal 904.
Where the fundamental wave has a frequency of f0 (=1/λ), the series resonant circuits 906a and 906b are configured so as to resonate with a frequency of 2f0, i.e., twice the frequency of the fundamental wave. Accordingly, second-order harmonics generated by the amplification elements 902a and 902b, which are in phase with each other, are canceled out in the series resonant circuits 906a and 906b. Therefore, the conventional power amplifier 9 shown in FIG. 6 is able to suppress generation of second-order distortion.
FIG. 7 is a diagram showing a structure of a conventional push-pull type power amplifier including a matching circuit with consideration of a harmonic impedance (see, for example, Japanese Patent Laid-Open Publication No. 5-29851). In the conventional power amplifier shown in FIG. 7, a fundamental wave inputted into an input terminal 911 is separated by a phase inversion circuit 912 into two signals inverted in phase with respect to each other. The two signals are outputted from the phase inversion circuit 912 through matching circuits 913 and 914 to FETs 915 and 916, respectively.
The FET 915 has a drain connected to a distributed constant line 917, and the FET 916 has a drain connected to a distributed constant line 918. A stab 919 having a length of L2, which corresponds to a ¼ wavelength of the fundamental wave, is connected to the distributed constant line 917 at a point at a distance of L1, which corresponds to an integral multiple of a ¼ wavelength of the fundamental wave, from an end connected to the FET 915. A stab 920 having a length of L2, which corresponds to a ¼ wavelength of the fundamental wave, is connected to the distributed constant line 918 at a point at a distance of L1, which corresponds to an integral multiple of a ¼ wavelength of the fundamental wave, from an end connected to the FET 916. Each of the stabs 919 and 920 has one end short-circuited, and acts as a short-circuit for an even-ordered harmonic.
Further, in order for the distributed constant lines 917 and 918 to be open for a third-order harmonic, a capacitor 921 is connected between the distributed constant lines 917 and 918 at points at a distance of L3, which corresponds to a 1/12 wavelength of the fundamental wave, from either of the ends respectively connected to the FETs 915 and 916. Moreover, a capacitor 922 is connected between the distributed constant lines 917 and 918 in order to achieve an impedance match with the fundamental wave in a portion between a connection point to the capacitor 921 and a connection point to either of the stubs 919 and 920. In a phase inverter 923, signals from the distributed constant lines 917 and 918 are brought into phase with each other, and combined into a signal which is outputted from an output terminal 924.
In the conventional power amplifier shown in FIG. 7, if gate voltages of the FETs 915 and 916 are set so as to be at a pitch-off point and large input signals are applied to the FETs 915 and 916, a drain voltage vd and a drain current id of each of the FETs 915 and 916 show waveforms as shown in FIG. 8. Specifically, a waveform of the drain voltage vd has a rectangular shape and includes fundamental wave components and odd-ordered harmonic components. A waveform of the drain current id has a half-wave rectified shape and includes fundamental wave components and even-ordered harmonic components. Thus, the drain voltage vd and the drain current id do not exist simultaneously in an amplification element, so that substantially no power is consumed by the amplification element, whereby it is possible to achieve substantially a 100% efficiency.
In the conventional power amplifier shown in FIG. 6, in order to use an impedance as a short-circuit for a second-order harmonic, it is necessary to provide the series resonant circuits 906a and 906b to a pair of FETs.
In the conventional power amplifier shown in FIG. 7, in order to use an impedance as a short-circuit for an even-ordered harmonic, it is necessary to provide the stabs 919 and 920 to a pair of FETs.
Thus, a circuit size is increased in each of the above-described conventional power amplifiers.
In each of the above paired series resonant circuits and the above paired stabs, both elements of the pair are required to have the same characteristic. However, in an amplifier modularized by means of integration, paired series resonant circuits or paired stubs may have high frequency characteristics slightly different from each other due to influence of a part adjacent thereto, for example. This might cause variation of a gain characteristic of the conventional power amplifier.